This invention relates to an improved light source carrier for a photoelectric position measuring system for measuring the relative position of two objects, of the type comprising a graduation carrier, adapted to be coupled to a first object, a graduation defined by the graduation carrier, and a scanning unit adapted to be coupled to a second object, the position of which is to be measured, said scanning unit comprising a scanning plate and at least one photosensor mounted in the scanning unit to scan the graduation.
Photoelectric measuring systems are typically used to measure the relative position of two objects, such as two components of a processing machine. In such measuring systems, a graduation of a scale is scanned by a scanning graduation of a scanning plate and associated photoelectric scanning elements included in the scanning unit. A light source is provided in the scanning unit to illuminate the graduations of the scale and the scanning plate. Light produced by the light source is modulated by the graduations as a result of their relative movement, and this modulated light is detected by the photoelectric scanning elements of the scanning unit, which generate periodic electric scanning signals. Preferably, the photoelectric scanning elements should be illuminated as uniformly as possible by the light source.
Often when a commercially available light emitting diode is used in the light source, not all of the light is directed towards the graduations. Rather, a portion of the light emerges laterally from the light emitting diode and can be reflected from the carrier of the light emitting diode or other components present in the environment. The reflection of such laterally directed light in the direction of the graduations of the scale and of the scanning plate can give rise to a considerable deterioration of the degree of modulation of the periodic scanning signals. A diaphragm or blind that is arranged on the surface of the photodiode through which the light passes can, of course, improve the degree of modulation. However, this approach can result in a severe diminution of the amplitude of the scanning signals.